Audion amplifier



Jllile 1 935. P. o. FARNHAM ET AL 2,005,080

AUDI ON AMPLIFIER Filed Oct. 5, 1929 4 Sheets-Sheet 1 M/craam oeres Rafe arren/ gwntou:

I SHKVI new.

June 18, 1935. P. o. FARNHAM ET AL AUDI ON AMPLIFIER Filed Oct. 5, 1929 4 Sheets-Sheet 2 iln manic;

June 18, 1935.

P. o. FARNHAM ET AL r 2,005,080

AUDION AMPLIFIER I v Filed Oct. 3, 1929 4 Sheets-Sheet 3 Jfaye Gain E/E,

6creen Grid l o/fage p Z W I 8H0: new,

June 18, 1935: R O FARNHAM ET AL 2,005,080

AUDI ON AMPLIFIER Filed Oct. 3, 1929 4 Sheets-Sheet 4 m in the n e i n:..

nected in "the amplified output from the audions being com- Patented- June 18, 1935 a AUDION AMPLIFIER Paul .0. Far-nham. and Raymond .Asserson; Boom mam. J-, a s n r y. to. Radio Corporation of 1 f Delaware 1929, Serial No.- 391,123;

N, Y5: a corpor Application October 3,

mesne. assignments, me iva,. w ork 13, Claims. (clam-471) This invention relates to audion amplifiers and particularlyto methods of and I apparatus for avoiding distortion when audion amplifiers are so operatedas to effect amplification of a relatively low order-- of magnitude.

The problem of volumecontrol in radio freduency-amplifiers-is usually complicated by the introduction-ofunwanted-effects, such as, for

example, a decrease in selectivity as the sensi- "tivity'is-decreased; .detun-ing of one or more stages of the amplifier, and the introduction-of distortion inthe modulation-of the impressed carrier. In cascaded amplifiers in which-the volume controlis based uponan adjustment of the control gridbias, or the screen-grid voltage in the case of tetrodes, distortion in'modulation occurs when the sensitivity-is reduced, for strong 1 signals of the order of 0.2 volt-or larger, to maintain a constant output;- Tq keep the impressed signal strength belowthe point at which appreciable distortionarises, ithas been the practice toreducethe signal 'voltage'derived' from the antenna by-some form of manual control, such'as a potentiometer.

Objects of the present invention-are to provide a method of and an audion stage for amplifying carrier wave signals; the method and apparatus being characterized by an absence of distortion in modulation when the amplification is reduced to a low value.

A- further object is to provide an amplifier stage capable of operating as a substantially linear amplifier when signal energy of the order of several volts'isimpressed upon the stage 'andhthe sensitivity of the stage has been reduced to alow valuel- I More particularly an objectisto provide an amplifier stage including a pair of audions conparallel across the, stageinput v circuit,

bined in phase opposing relation whereby the stage amplification is, determined by. the difference 'between the amplification rates of the two audions. i I

' These and other objects of the invention will be a p ren from, h l w nsu p fi a qn h ak nw t the co pa in d awin s in which a Fig; 1-isanelectricaldiagrarn of onecircuit Fig 2 is a static haracteristic cf one audion,

hebu f howingp at current asi i n t o of Qatr rid b as,

Fig. 3 is a series of similar static characteristic the relationship between stage'gain anddiflerences in control grid biases *for the-stage shown 6 in Fig. 1-; V

Fig. 5 comprises a similar curve "showing the relationship between istage gain and differences in-screen grid voltage in an amplifier of the type shown in Fig. 6; a

Fig. 6 is a circuit diagra of an amplifier stage in which volume controlis effected by-- adjustment of screen-grid voltages; and 3 Fig. '7 is:a circuit diagram of another embodiw ment of theinventionn v I 15 In the drawings, the reference numerals l and 2 indicate the two audions'which, withthe associated impedance l network; form an amplifier stage in which the signal energy. isimpressed upon the .two audions-in parallel, but-the amplified "gg outputs from thetubes are combined in push-pull, or phaseopposing relationship. I

. As shownin the drawings, the. inputcircuitfor the stage comprises. an-inductance 3; coupled :to an inductance 4 in the. antenna l circuit, and 25 shunted by a variable condenser 5 fontuning over a band of frequencies..- The, controlg'ridsGv-of both .audions are connectedto the! high potential terminalbfthe tuned circuit. 3,.5 =and the..low I v potential ,terminalis: grounded in the usual man- 30 ner to the chassis or casing ofthe amplifier. The output circuit; of the stage isprovided. by. an inductance 6;.shunted by a tuning, condenser], the

inductance beingcoupled to. the inductances 8f,

8", mineanqdec c s f aucti n I andclrreas i j h b ttery ein ma e; o ac n aly-tap '40 the.-, Q ththiaanen m ntth e rn tin m r t qwi s. in d aeqesd' 8 i in uce rrents o pp phase in he S a e p nu indu tees? atne nap tna aim-st .i se e m d by a ffie z ce in; he c rre t win th two imic ndutans=e I a The positive, potential for, the screen grids G2 mash 'ibbi ine limm.t ab tt r pre er l by. meansof an, adj istable tap I Land. the usual by pass condensers fll are pr ovi ded for-the batmleaas and. b tween h fia h an ground; vetmlm m ifica n, ed by. r ulats. thesm de db ases i re e ,upqn a di- Oils '1 and]; Individual grid bias batteries l2, on

about the quiescent point, the

The method employed will be apparent from-a consideration of the curves of Figs. 2 and 3, The static characteristic ship between control grid-:bia'sf Ec .and..plate current Ip for a particular tetrode of .thecommercial type known as UY224,

plate. The actual dynamic curve {or atetrode of high plate impedance is not identical with the static characteristic but is a curve of the samesponding plate current I -Fig. 2, amplification is proportional to the slope of the characteristic and may be controlled by regulating the grid bias to place the operating point at any desired point on the characteristic. Thisgeneral method of control is well known as employed to effect control of volume.

While the wave shape of I corresponding to operation at -2 volts grid bias is symmetrical corresponding to low amplification at"4.5 volts grid bias is not symmetrical and indicates distortion in the modulation.

In the circuit of Fig- 1,.the effective output is not determined by the amplification of either. tube, but by the difierence in their. amplification. rates since the amplified signal energy is combined in phase opposing relation. The magnitudes of the alternating current. voltages in :the respective plate circuits dependupon the position oftheoperating point on the. characteristic curve! of each tube,.and this position is determined by the control grid bias of the respective tubes.

The effective amplification is therefore deter- :mined by the difference in slopes of the characteri'stic curves at the respective operating points.

The net plate current curve for the stage may-be determined graphically in the manner indicated in Fig. 3. Sol-id'linecurves A, 'B are the static characteristics for audions I and-2,respectively'; the curves B1, B2, etc. being duplicates -of-curve B,'- but displacedtherefrom -by' distancescorresponding to 1 volt, 2 volts, etc., respectively, on the scaleof abscissa. Net plate current curves C2, C1,-etc., maybe plotted to show t e relationship between the effective plate current ofth'e stage and grid bias when the volume control-is so adnegative thanthat on volts, 1 volt, etc.,respectively.

A? mathematical analysisgwill show that, in

general, the difierence curves are of an-order one power lower than the characteristic curves.

The difierenceor net plate curves indicate that distortion in modulation may be substantially eliminated evenwhen strong signals are received,

. by locating the operating points .01 the audions in regions on the respectivecurves @which'correspond to linear response regions on the net plate current curve, ,the sta e gain being reduced to a curve A shows the relation when operated with 754' volts on the screen-grid and 180 volts on the" has been previously Y wave shape for 1 low value by making thebias on one tube but slightly difierent from that on the other tube.

The stage gain may be regulated by placing a fixed bias on onetube and adjusting the bias on the second tube, or thebias voltages may be simultaneously adjusted. When both voltages are adjusted, we prefer to keep the sum' of the biasesconstant' as their respective :fvalues are varied The curves of Fig. 4 represent the relation between stage gain and bias voltage differences for t e amplifier circuit shown in Fig. 1, when emplaying UY224 tubes with screen-grid and plate potentials of 75 and 180 volts respectively. Curves D and'E were obtained when the bias voltages on audion I, wereconstant and equal, respectively to -1.5 and '6' volts. The data for curve F was obtained when the sum of the bias voltages EC and Ecf on the tubes was maintained constant at 10 volts, while the individual biases were adjusted.

7 v If the operating characteristics of therespeetive tubes and their associated networks were identicaLthe stage gain in each instancewould fact that the gain curves do not fall to zero at were notidenticala The inventionis equally applicable to volume control systems in which the emission, or; the plate or screen grid potentialsare varied to control theamplification. ,1.

Foiz-egrample the circuit of 'Fig. 1 may be modified to permitthe application of v(iiiferentjscreengrid voltagesto the respective audions.

An amplifierjstage ofthis type is illustrated in Fig. 6, in which the separate bias batteries I2,

I21 are replaced by acomm on bias battery I2, and

individual leads and adjustable taps [0,10", re-

spectively, are provided for applying. positive potentials to the screen-grids of audions I and 2,

I The curve Gof Fig. v 5 showsthe variation-pf stage gain as the screen-grid voltage of,,one tetrode ,was varied,thescreen gridvoltage. on the other tetrode remaining fixed at 75 volts, and the [bias andtplate voltages on, b th tetrodes being 7 1.5 and. 18Q volts, respectively. The operating .range for volume ,control will, of .course, in-

clude but one branch of" the curve for the. particular tubesand circuitfrom which the .plotted data was obtained, the controlirange corresponds to variation 2 potential between zero, and 53 volts.

It is not essential that the signal energy. be

of the second screen-grid impressed across the pair ofaudions in parallel sincethe same combination of amplified energy in phase opposingrelation. may be effected when thepotentials onthe control grids are o fopposite phase and both1audionsfeedthe stage output circuit in parallel I justed that the bias voltage on audion'2 is more i As shown in Fig. 7,, an amplifierstage employing thisrnethod'may includean inputtrans 'former having a primary 4 across which the-signal voltage E0 is developed, and a secondary 3',

the f opposite ends of the secondary; connected to the respective control gridsG and the center poi'nt'of' the secondary being grounded.

The: plate circuits of audions I,.2 have-a common amplified energy impedance 8 for transferring to the stage output circuit 6, I. The remaining circuit elements of the stagemayfbe of appropriate arrangement and magnitude for Ieifecting control of the amplification of one or both audions fall to zero when the tube biases were equal. The e of a cascaded amplifier, circuits embodying the invention may be used in subsequent stages, or in a plurality of the stages of an amplifier.

When employed for the amplification of unmodulated carrier waves, low amplification may be effected without the introduction of harmonics.

We claim: I

1. In audion amplifiers of the type in which distortion occurs at low amplification and in which means are provided for controlling over a wide range the amplitude of pulsations in the output circuit resulting from pulsations impressed upon the input circuit of the amplifiers, means for effecting linear amplication throughout the controlling range, which comprises means for impressing the signal energy across the input terminals of a pair of audions, means for adjusting the energizing potentials upon the respective audions to effect non-linear amplification at different rates, and means for delivering the amplified outputs from said audions to a common output circuit in phase opposing relation.

2. In a system for effecting undistorted amplification control of carrier waves over a wide range of intensities, means for impressing the signal in parallel upon two audions, means for impressing the amplified output from said audions upon a common output circuit and in phase opposing relation, and means for adjusting the amplification rates of the individual audions to effect the desired amplification.

3. In audion amplifiers of the type in which modulation distortion occurs at low amplification, and in which means are provided for controlling over a wide range the amplitude of pulsations in the output circuit resulting from pulsations impressed upon the input circuit of the amplifiers, means for effecting low amplification without distortion which comprises means for impressing the signal in parallel, upon two audions, means for adjusting amplification control voltages of the respective audions to effect amplification within the square law range of operation but at different rates, and means for combining the amplified, outputs from the respective audions in phase opposing relation.

4. The invention as set forth in claim 3, wherein the control grid bias voltage adjusting means is adapted to adjust the grid bias voltages in opposite sense to vary the amplification rates of the respective tubes.

5. The invention as set forth in claim 3, wherein the control grid bias voltage adjustingmeans is adjusted to adjust the respective voltages in opposite sense to vary the amplification rates of the respective tubes, and in such a way that the sum of the said bias voltages being maintained constant as the separate bias voltages are varied.

6. In a carrier wave amplifier, the combination with an input circuit, an output circuit, and a pair of audions of twosets of impedance elements coupling the respective circuits with said audions, one of said set of impedance elements being arranged to establish the flow of currents in phase opposing relation between its associated circuit and the respective audions, and the second set of impedance elements being arranged to establish the flow of currents of the same phase between its associated circuit and the respective audions; and means impressing upon said audions energizing potentials of such magnitudes that i both audions operate upon curved portions of their respective characteristic curves.

'7. In an amplifierstage having an input circuit and an output circuit, means for controlling the amplitude of pulsations in the output circuit resulting from pulsations impressed upon the input circuit thereof over a wide range comprising a pair of audions, circuit elements connecting the input terminals of said audions in parallel across an input circuit, anode circuits individual to the respective audions, an output circuit to which said anode circuits are coupled in phase opposing relationship, and means impressing upon the respective audions energizing voltages of such different and related magnitudes that substantially linear amplification is effected by said stage.

8. The invention as set forth in claim 7, in combination with means for adjusting an energizing voltage on one audion to control the net amplification of said stage.

9. The invention as set forth in claim '7, in combination with means for simultaneously adjusting energizing voltages on the respective audions to vary the amplification ratios thereof in opposite sense.

10. In an amplifier stage having an input circuit and an output circuit, means for controlling the amplitude of pulsations in the output circuit resulting from pulsations impressed upon the input circuit thereof over a wide range comprising a pair of audions, an input circuit connected to said audions to energize the same in parallel,

coupling impedances individual to the anode circuits of the respective audions, an output circuit to which said coupling impedances are coupled in opposing relationship, and means impressing upon each of said audions amplification controlling voltages of different magnitude and of such value that both of said audions operate upon curved portions of their respective characteristic curves.

11. The invention as set forth in claim 10, in

combination with means for adjusting the control voltage impressed upon one of said audions.

12. The invention as set forth in claim 10, in combination with means for sense, the control voltages impressed upon said audions,

13. The steps in a method of controlling the degree of amplification of signalling energy which comprise, utilizing a portion of the signalling energy to produce a non-linearly amplified related electrical energy output, utilizing another portion of the signalling energy to produce a nonlinearlyamplified electrical energy 0 ferent intensity than the first named output and combining the two non-linearlyamplified outputs in phase opposition so cation of the signal energy is determined by the difference in the intensity of each of the produced outputs. I

7 PAUL O. FARNHAM.

RAYMOND ASSERSON' varying in opposite utput of difjthat the effective amplifie 

